Electronics Cookbook

Chapter 17. Operational Amplifiers

17.0 Introduction

Operational amplifiers, or more commonly known as op-amps, have a helpful way of making theory easy to implement. If you need to make a filter or preamplifier, an op-amp is often the best solution.

An op-amp has two inputs and one output and is represented in schematic diagrams (Figure 17-1) as a triangle with the output coming from one apex and the two inputs labeled + and – on the opposite side. They also need positive and negative power connections. They are available in a variety of IC packages with 5, 6, or 8 pins. ICs are also available that contain two or four op-amps in a single package.

Figure 17-1. Schematic Symbol for an Op-Amp

Op-amps amplify the difference between the + and – inputs. The gain of an amplifier will generally be millions or even billions. So the output voltage may be 1,000,000 times the difference between the input voltages. Such a high gain may sound desirable, but it is actually too high to do anything useful with. To reduce the gain to a more manageable level, an op-amp is almost always used with negative feedback where some of the output is fed back to the negative input. You will find examples of using feedback in this way in Recipe 17.4 and Recipe 17.5. In Recipe 17.6 you will see how if all of the output is fed back to the negative input the output will follow the positive input to the op-amp ...